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We investigate suspended particles collected from the upper mixed layer in the inner shelf of the southern East China Sea during autumn 2013 for carbon and nitrogen (POC and PN) contents and their isotope compositions (δ13C and δ15N) along with hydrographic parameters to understand the sources and dynamics of particulate organic matter (POM) in the study area. Results indicated that the extensive hydrological processes affect the biogeochemical composition of suspended POM, as revealed by the horizontally mixing POM and the spatial variation of δ15N and molar C/N ratio. Low C/N (2.4–6.5) and a weak correlation between POC and in situ chlorophyll fluorescence suggested that POM is dominated by the recently formed and well‐preserved planktonic OM. By fitting the linear correlation between δ13C and POC data with a photosynthetic fractionation model, we further disentangled dynamic controls of phytoplankton production and species diversity on δ13C variability (−24.3 to −21.3‰), emphasizing the constant effect of productivity‐derived POC on δ13C (0.02‰ per μg/L). The δ15N variability (2.3–7.4‰) is largely controlled by the mixing of isotopically different nitrogen sources, in which the importance of biological nitrogen fixation is unfolded based on the small δ15N and the negative correlation between δ15N of POM and seawater temperature. This implies that Kuroshio‐induced biological N fixation plays an important role in supporting the marine production in the East China Sea. These hydrologically driven δ13C and δ15N changes of marine productivity‐derived POM suggest that internal biophysical dynamics rather than terrestrial versus marine OM mixing largely control the C and N compositional variability in the shelf seas. Plain Language Summary: Mixing of water masses on the shallow continental shelf of the East China Sea is thought to impact biogeochemical dynamics of suspended particulate organic matter on seasonal basis, but the mechanisms involved remain unclear. In this study, we measured carbon and nitrogen elemental and isotopic composition of particulate organic matter in the upper mixed layer along with hydrographic parameters during autumn season when the upwelled Kuroshio water travels closer to the coast. Results indicated that particulate organic matter is dominated by two sources of plankton‐derived organic matter that mixed horizontally by hydrodynamics. In addition, the upwelled Kuroshio contribute to marine productivity significantly by inducing biological nitrogen fixation on the shelf; the region is otherwise N‐starved. These findings based on observational and analytical results can help marine biogeochemists understand how carbon and nitrogen isotopes varied seasonally by the impact of offshore current over the river‐dominated shallow shelf, although the influence of Kuroshio on marine fisheries is beyond the scope of our investigation. Key Points: Hydrological conditions largely control the biogeochemical composition of suspended particulate organic matter in the southern East China SeaParticulate organic matter in the upper mixed layer is mainly of marine origin, despite having wide ranges of δ13C and δ15N valuesKuroshio‐induced biological nitrogen fixation is important for marine production in the southern East China Sea [ABSTRACT FROM AUTHOR]

Continental shelves and marginal seas are key sites of particulate organic matter (POM) production, remineralization and sequestration, playing an important role in the global carbon cycle. Elemental and stable isotopic compositions of organic carbon and nitrogen are thus frequently used to characterize and distinguish POM and its sources in suspended particles and surface sediments in the marginal seas. Here we investigated suspended particulate matter (SPM) collected around deep chlorophyll maximum (DCM) layers in the southern East China Sea for particulate organic carbon and nitrogen (POC and PN) contents and their isotopic compositions (ϐ13CPOC and ϐ15NPN/ to understand provenance and dynamics of POM. Hydrographic parameters (temperature, salinity and turbidity) indicated that the study area was weakly influenced by freshwater derived from the Yangtze River during summer 2013. Elemental and isotopic results showed a large variation in ϐ13 CPOC (-25.8 to -18.2 ‰) and ϐ15NPN (3.8 to 8.0 ‰), but a narrow molar C =N ratio (4.1-6.3) and low POC = Chl a ratio (<200 g g-1) in POM and indicated that the POM in DCM layers was newly produced by phytoplankton. In addition to temperature effects, the range and distribution of ϐ13CPOC were controlled by variations in primary productivity and phytoplankton species composition; the former explained ∼70% of the variability in ϐ13CPOC. However, the variation in ϐ15NPN was controlled by the nutrient status and ϐ15NNO-3 in seawater, as indicated by similar spatial distribution between ϐ15NPN and the current pattern and water masses in the East China Sea; although interpretations of ϐ15NPN data should be verified with the nutrient data in future studies. Furthermore, the POM investigated was weakly influenced by the terrestrial OM supplied by the Yangtze River during summer 2013 due to the reduced sediment supply by the Yangtze River and north-eastward transport of riverine particles to the northern East China Sea. We demonstrated that the composition of POM around DCM layers in the southern East China Sea is highly dynamic and largely driven by phytoplankton abundance. Nonetheless, additional radiocarbon and biomarker data are needed to reevaluate whether or not the POM around the DCM water depths is influenced by terrestrial OM in the river-dominated East China Sea. [ABSTRACT FROM AUTHOR]